The invention disclosed herein relates generally to offset correction for electrically powered sensors whose outputs may exhibit undesired offsets due to fabrication process variations, and more particularly to a method and circuit for providing offset correction which accommodates the temperature dependent sensitivity of an integrated circuit Hall effect or similar sensor.
Hall effect sensors are recognized as offering substantial advantages in many sensing applications. For some time, such sensors have been widely used where an on-off or binary output is required or acceptable. Hall effect sensors have also been used in various applications requiring analog outputs. However, such uses have been limited because the output voltage of a Hall element is so low that amplification is required, and, since the Hall element sensitivity varies with temperature, the amplification must be made to vary with temperature in a compensating manner. Further, since it is preferred that any amplifier used with the Hall element operate about the center of its operating range, provisions for offset correction are required. Compensation for these characteristics can be provided with suitable compensation circuitry. However, the task is complicated by the requirement for precise temperature dependency relationships. In particular, the sensitivity of a Hall element is quite temperature dependent. Thus, in order to achieve a linear output in applications in which the temperature is not constant, it is necessary that the associated amplifier exhibit a complementary temperature dependent gain characteristic.
The offset exhibited by a Hall element appears as a nonzero output voltage when no magnetic field is applied. It is known that this offset can be cancelled by causing electric currents of the proper magnitudes to flow through the output terminals. However, the proper magnitudes of currents also depend on temperature. In order to achieve compensation, the temperature dependency of these currents must track the temperature dependency of the Hall effect device. A further requirement is that these currents be adjustable to permit trimming of the device to a zero or other desired predetermined offset after manufacture since it cannot be predicted beforehand what the offset will be.
One approach to providing offset compensation is to control the currents through the Hall element output terminals by means of a control signal having the same temperature dependency as the Hall effect device. Such a control signal can be generated by a resistor having the same temperature characteristics has the Hall effect device supplied with power from the same source. Where the Hall effect device is part of an integrated circuit and is formed in an epitaxial layer, a resistor for producing the offset current control signal can be formed advantageously in the same epitaxial layer, thus insuring that both the Hall effect device and the resistor have substantially identical temperature dependent characteristics. However, such construction results in a problem, since such a resistor cannot be trimmed after it is formed to the value required for eliminating any offset which may exist.
The applicant has devised a unique integrated circuit Hall effect device in which the Hall effect element and resistor for producing a contol current are formed in the same epitaxial layer, and which also permits precise adjustment of currents through the Hall effect element output terminals.